fix(sdk): entity picker theming fixes (#50449) (#50590)

* use css variables for skeleton background color

* apply theme to folder color

* replace theme.white with themeColor

* use name of folder chevron color

* add background-inverse color mapping

* Revert "add background-inverse color mapping"

This reverts commit d0b546c5.

* avoid overriding folder chevron color in emotion

* use Mantine style overrides for NavLink

Co-authored-by: Phoomparin Mano <poom@metabase.com>